This comes from the introduction to the Alice Bailey/Djwhal Khul book "The Externalization of the Hierarchy" which can be found at http://www.bibliotecapleyades.net/sociopolitica/externalisation/contents.html
Meditation involves the living of a one-pointed life always and every day. This perforce puts an undue strain on the brain cells for it brings quiescent cells into activity and awakens the brain consciousness to the light of the soul. This process of ordered meditation, when carried forward over a period of years and supplemented by meditative living and one-pointed service, will successfully arouse the entire system, and bring the lower man under the influence and control of the spiritual man; it will awaken also the centers of force in the etheric body and stimulate into activity that mysterious stream of energy which sleeps at the base of the spinal column. When this process is carried forward with care and due safeguards, and under direction, and when the process is spread over a long period of time there is little risk of danger, and the awakening will take place normally and under the law of being itself. If, however, the tuning up and awakening is forced, or is brought about by exercises of various kinds before the student is ready and before the bodies are coordinated and developed, then the aspirant is headed towards disaster. Breathing exercises or pranayama training should never be undertaken without expert guidance and only after years of spiritual application, devotion and service; concentration upon the centers in the force body (with a view to their awakening) is ever to be avoided; it will cause over-stimulation and the opening of doors on to the astral plane which the student may have difficulty in closing. I cannot impress too strongly upon aspirants in all occult schools that the yoga for this transition period is the yoga of one-pointed intent, of directed purpose, of a constant practice of the Presence of God, and of ordered regular meditation carried forward systematically and steadily over years of effort.
When this is done with detachment and is paralleled by a life of loving service, the awakening of the centers and the raising of the sleeping fire of kundalini will go forward with safety and sanity and the whole system will be brought to  the requisite stage of "aliveness." I cannot too strongly advise students against the following of intensive meditation processes for hours at a time, or against practices which have for their objective the arousing of the fires of the body, the awakening of a particular center and the moving of the serpent fire. The general world stimulation is so great at this time and the average aspirant is so sensitive and finely organized that excessive meditation, a fanatical diet, the curtailing of the hours of sleep or undue interest in and emphasis upon psychic experience will upset the mental balance and often do irretrievable harm.
Let the students in esoteric schools settle down to steady, quiet, unemotional work. Let them refrain from prolonged hours of study and of meditation. Their bodies are as yet incapable of the requisite tension, and they only damage themselves. Let them lead normal busy lives, remembering in the press of daily duties and service who they are essentially and what are their goal and objectives. Let them meditate regularly every morning, beginning with a period of fifteen minutes and never exceeding forty minutes. Let them forget themselves in service, and let them not concentrate their interest upon their own psychic development. Let them train their minds with a normal measure of study and learn to think intelligently, so that their minds can balance their emotions and enable them to interpret correctly that which they contact as their measure of awareness increases and their consciousness expands.
Students need to remember that devotion to the Path or to the Master is not enough. The Great Ones are looking for intelligent cooperators and workers more than They are looking for devotion to Their Personalities, and a student who is walking independently in the light of his own soul is regarded by Them as a more dependable instrument than a devoted fanatic. The light of his soul will reveal to the earnest aspirant the unity underlying all groups, and enable him to eliminate the poison of intolerance which taints and hinders so many; it will cause him to recognize the spiritual fundamentals which guide the steps of humanity; it will force  him to overlook the intolerance and the fanaticism and separativeness which characterize the small mind and the beginner upon the Path, and help him so to love them that they will begin to see more truly and enlarge their horizon; it will enable him to estimate truly the esoteric value of service and teach him above all to practice that harmlessness which is the outstanding quality of every son of God. A harmlessness that speaks no word that can damage another person, that thinks no thought which could poison or produce misunderstanding, and which does no action which could hurt the least of his brethren - this is the main virtue which will enable the esoteric student to tread with safety the difficult path of development. Where the emphasis is laid upon service to one's fellowmen and the trend of the life force is outward to the world, then there is freedom from danger and the aspirant can safely meditate and aspire and work. His motive is pure, and he is seeking to decentralize his personality and shift the focus of his attention away from himself to the group. Thus the life of the soul can pour through him, and express itself as love to all beings. He knows himself to be a part of a whole and the life of that whole can flow through him consciously, leading him to a realization of brotherhood and of his oneness in relation to all manifested lives.